As shown in FIG. 1, a conventional electric generator 10 comprises a generally cylindrical rotor 12 carrying axial field or rotor windings (also referred to as rotor turns) 13 about the circumference thereof. Current flow through the field windings 13 generates a magnetic field within a stationary armature or stator 14. One end 15 of the rotor 12 is drivingly coupled to a steam or gas-driven turbine (not shown in FIG. 1) for supplying rotational energy to turn the rotor 12 and causing rotation of the magnetic field produced thereby. According to known dynamoelectric principles, the rotating magnetic field of the rotor 12 induces current flow in stator windings 18 (only one stator winding 18 is illustrated in FIG. 1) of the stator 14. An end 16 of the rotor 12 is coupled to an exciter (not shown) for supplying direct current to the rotor windings 13 to produce the aforementioned magnetic field.
The stator 14 has a generally cylindrical shape and an annular traverse cross section defining a longitudinal bore extending there through for accepting the rotor 12, which thereby extends substantially the longitudinal length of the stator 14. The stator 14 comprises a core 17 further comprising a plurality of thin, generally annular high-permeability laminations disposed in side-by-side orientation. Each lamination comprises an insulative coating such that two adjacent laminations are electrically insulated to reduce eddy current flow within the core 17. The alternating current generated in the stator windings 18 by action of the rotating magnetic field of the rotor 12 is conducted to main generator leads 19 for supplying the generated current to an external electrical load. The stator windings 18 are connected through end turns 20. A gap 21 formed between the rotor 12 and the stator 14 typically measures between about one and two inches.
The rotor 12 and the stator 14, and other generator components not directly relevant to the present invention, are enclosed within a frame 23.
FIG. 2 illustrates rotor axial leads 54 through which the direct current generated by the exciter is coupled to the rotor windings 13. A conventional two-pole rotor comprises eight to eighteen axial slots each carrying a plurality of mutually insulated conductive bars that are symmetrically oriented along the rotor circumference. For example, one rotor design comprises fourteen slots such that one rotor pole comprises seven slots. The conductive bars comprising a pole are interconnected by end turns, i.e., arcuate winding segments located at a rotor end for connecting the axially disposed conductive bars. The end turns are physically restrained by retaining rings 56.
FIG. 3 is a cross-sectional view of the stator 14, illustrating a face 60 of one lamination of the stator core 17 and circumferential stator slots 62 defined by inwardly-directed circumferential teeth 64 extending axially along the stator 14. The stator windings 18 are disposed within the slots 62 between two adjacent teeth 64. Bolt/nut combinations 66, or similar fasteners, extend axially through and secure the laminations to form the stator core 17.
As illustrated more clearly in the partial view of FIG. 4, the stator core 17 comprises a plurality of stacked laminations 67. Two adjacent stator teeth 64 (only one is shown in FIG. 4) formed in the laminations 67 define stator slots 62 for receiving the stator windings 18. Typically, two stator windings 18 are disposed within each slot 62. The two stator windings 18 are retained within the slot 62 by a shim 70, a ripple spring 72 and a wedge 74 having opposed beveled edges 76 for engaging correspondingly shaped grooves 80 in sidewalls 81 of the teeth 64. The ripple spring 72 is compressed between the wedge 74 and the shim 70 to apply a radially outwardly directed force retaining the two stator windings 18 in place within the slot 62.
It is known that a shorted rotor winding turn reduces the magnetic flux and modifies the power dissipation profile of the rotor windings. Such changes in power dissipation can produce non-uniform heating of the rotor windings, resulting in thermally induced rotor distortion and vibration that can damage the rotor and other generator components. The vibrations may also excite the natural resonant frequencies of generator components, including the pad on which the generator is mounted. Pad vibration can lead to severe damage to the generator.
The primary cause of a short circuit is the breakdown of insulation separating the conductor bars or end turns of the rotor, by wearing of the rotor winding insulation or slot cell insulation that separates the rotor windings. The short circuits may occur only when the rotor is at rest or only when the rotor windings and end turns are subjected to centrifugal forces caused by rotation. The former type of short circuit can be detected by known static tests, while the latter can be detected only while the rotor is turning at or near its operational speed.
The considerable economic value of a generator and the high cost of replacing generated power during a generator service outage encourages an owner to continue operating the generator so long as operation is safe and will not likely damage the generator. A single shorted rotor turn can generally be tolerated, although it may be necessary for the operator to add balance weights to compensate the vibration levels under different operating conditions. If the vibrations cannot be eliminated by rotor balancing it may be necessary to shut down the generator and remove the rotor to determine the location of each short and repair the affected rotor winding. Also, if the rotor windings develop several shorts the short circuit current can flow through the rotor body. The rotor body may be damaged by this current flow and will thus require a substantial repair effort.
One technique for locating rotor short circuits uses a stator-mounted flux probe for measuring the rotor magnetic field flux, from which, using known techniques, the operator can determine the existence and location of rotor winding shorts. According to this prior art process, installation of the flux probe requires removal of the rotor to access the stator and mount the probe thereto. In one embodiment, the probe is bonded and pinned, using dowel pins for example, to a radially inwardly directed face 82 of the stator wedge 74 of FIG. 4. The rotor is then replaced and the generator returned to service. Once operational again, probe signals indicative of the rotor's magnetic field are analyzed to determine the location of the short circuit. Such a prior art probe installation process is an expensive and time consuming undertaking for the generator operator, requiring a shut down duration of several days. The cost of the shutdown, with respect to both the process for installing the probe and the cost for replacement power can be considerable. Once the location of the short is determined, the generator is again shutdown, the rotor removed and the necessary rotor repairs performed.